gsmtap-rlp: Add support for skipping generating NULL frames

If there's nothing to transmit over a CSD NT channel, both ends generate
NULL frames.  Let's add an option to suppress GSMTAP output for those,
creating pcap files with less noise.

Change-Id: I85a2159cfaa01bfb4205c1462e3a9dbda68e4bad
Depends: libosmocore.git I2d9bd8eb4f0cd0f72c436996767b199429596917
---
M TODO-RELEASE
M include/osmo-bts/bts.h
M src/common/l1sap.c
M src/common/vty.c
M tests/osmo-bts.vty
5 files changed, 38 insertions(+), 5 deletions(-)

diff --git a/TODO-RELEASE b/TODO-RELEASE
index 1ed7b53..2bbfab5 100644
--- a/TODO-RELEASE
+++ b/TODO-RELEASE
@@ -8,3 +8,4 @@
 # If any interfaces have been removed or changed since the last public release: c:r:0.
 #library	what	        description / commit summary line
 libosmogsm	>1.9.0		added new PRIM_INFO to include/osmocom/gsm/l1sap.h
+libosmogsm	>1.9.0		use of RLP code in libosmogsm
diff --git a/include/osmo-bts/bts.h b/include/osmo-bts/bts.h
index b12125c..c3e5dce 100644
--- a/include/osmo-bts/bts.h
+++ b/include/osmo-bts/bts.h
@@ -378,6 +378,7 @@
 		uint32_t sapi_mask;
 		uint8_t sapi_acch;
 		bool rlp;
+		bool rlp_skip_null;
 	} gsmtap;
 
 	struct osmux_state osmux;
diff --git a/src/common/l1sap.c b/src/common/l1sap.c
index c579af5..e9095de 100644
--- a/src/common/l1sap.c
+++ b/src/common/l1sap.c
@@ -33,6 +33,7 @@
 #include <osmocom/gsm/l1sap.h>
 #include <osmocom/gsm/gsm_utils.h>
 #include <osmocom/gsm/rsl.h>
+#include <osmocom/gsm/rlp.h>
 #include <osmocom/core/gsmtap.h>
 #include <osmocom/core/gsmtap_util.h>
 #include <osmocom/core/utils.h>
@@ -1855,6 +1856,7 @@
 {
 	struct gsm_bts_trx *trx = lchan->ts->trx;
 	struct gsmtap_inst *inst = trx->bts->gsmtap.inst;
+	struct osmo_rlp_frame_decoded rlpf;
 	pbit_t *rlp_buf;
 	int byte_len;
 
@@ -1908,6 +1910,12 @@
 		byte_len = osmo_ubit2pbit_ext(rlp_buf, 0, data, 0, data_len, 1);
 	}
 
+	if (trx->bts->gsmtap.rlp_skip_null) {
+		int rc = osmo_rlp_decode(&rlpf, 0, rlp_buf, byte_len);
+		if (rc == 0 && rlpf.ftype == OSMO_RLP_FT_U && rlpf.u_ftype == OSMO_RLP_U_FT_NULL)
+			return;
+	}
+
 	gsmtap_send_ex(inst, GSMTAP_TYPE_GSM_RLP, trx->arfcn | is_uplink ? GSMTAP_ARFCN_F_UPLINK : 0,
 		       lchan->ts->nr,
 		       lchan->type == GSM_LCHAN_TCH_H ? GSMTAP_CHANNEL_VOICE_H : GSMTAP_CHANNEL_VOICE_F,
diff --git a/src/common/vty.c b/src/common/vty.c
index b5ab132..2bc5bf8 100644
--- a/src/common/vty.c
+++ b/src/common/vty.c
@@ -447,8 +447,12 @@
 		sapi_buf = osmo_str_tolower(get_value_string(gsmtap_sapi_names, GSMTAP_CHANNEL_ACCH));
 		vty_out(vty, " gsmtap-sapi %s%s", sapi_buf, VTY_NEWLINE);
 	}
-	if (bts->gsmtap.rlp)
-		vty_out(vty, " gsmtap-rlp%s", VTY_NEWLINE);
+	if (bts->gsmtap.rlp) {
+		if (bts->gsmtap.rlp_skip_null)
+			vty_out(vty, " gsmtap-rlp skip-null%s", VTY_NEWLINE);
+		else
+			vty_out(vty, " gsmtap-rlp%s", VTY_NEWLINE);
+	}
 	vty_out(vty, " min-qual-rach %d%s", bts->min_qual_rach,
 		VTY_NEWLINE);
 	vty_out(vty, " min-qual-norm %d%s", bts->min_qual_norm,
@@ -2389,11 +2393,16 @@
 }
 
 DEFUN(cfg_bts_gsmtap_rlp, cfg_bts_gsmtap_rlp_cmd,
-	"gsmtap-rlp",
-	"Enable generation of GSMTAP frames for RLP (non-transparent CSD)\n")
+	"gsmtap-rlp [skip-null]",
+	"Enable generation of GSMTAP frames for RLP (non-transparent CSD)\n"
+	"Skip the generation of GSMTAP for RLP NULL frames\n")
 {
 	struct gsm_bts *bts = vty->index;
 	bts->gsmtap.rlp = true;
+	if (argc >= 1 && !strcmp(argv[0], "skip-null"))
+		bts->gsmtap.rlp_skip_null = true;
+	else
+		bts->gsmtap.rlp_skip_null = false;
 	return CMD_SUCCESS;
 }
 
diff --git a/tests/osmo-bts.vty b/tests/osmo-bts.vty
index a7be40c..97629d0 100644
--- a/tests/osmo-bts.vty
+++ b/tests/osmo-bts.vty
@@ -262,7 +262,7 @@
   gsmtap-sapi (enable-all|disable-all)
   gsmtap-sapi (bcch|ccch|rach|agch|pch|sdcch|tch/f|tch/h|pacch|pdtch|ptcch|cbch|sacch)
   no gsmtap-sapi (bcch|ccch|rach|agch|pch|sdcch|tch/f|tch/h|pacch|pdtch|ptcch|cbch|sacch)
-  gsmtap-rlp
+  gsmtap-rlp [skip-null]
   no gsmtap-rlp
   osmux
   trx <0-254>

